To meet industry needs for electrical power converters which convert AC to DC without injecting large amounts of harmonic currents into the power system, numerous transformer and converter topologies are available, some of which are described in the documents referenced above. Methods whereby three-phase power is converted to nine-phase power before processing, are described in several U.S. patents. For many industrial applications a small step down of voltage is desirable. Cost-effective designs used in the variable frequency drive industry, are cited in the references above. Continual pressures to reduce losses and cost, motivate creation of new autotransformer methods. A polygon transformer such as described by Zou et. al., in 2002 requires only five windings per phase, but has the disadvantage of having a large circulating third harmonic current. This significantly increases the transformer kVA rating. The 2007 patent described by Paice overcomes this disadvantage while still retaining five windings per phase.
The invention presented here is a new design that obtains further improvements in efficiency and flexibility by using six windings per phase.